Electrochemical sensors are known and can be used to detect various types of gases including oxygen as well as toxic gases such as carbon monoxide, sulphur dioxide and hydrogen sulfide. Representative sensors have been disclosed in U.S. Pat. No. 5,668,302 to Finbow et al. entitled “Electrochemical Gas Sensor Assembly”, issued Sep. 16, 1997, and U.S. Patent Application No. 2010/0252455 published Oct. 7, 2010 and entitled “Methods of Operation of Electrochemical Gas Sensors”. Both the '302 patent and the '455 application are commonly owned with the present application and are incorporated herein by reference.
To improve their usefulness, it is desirable that they function as expected. To monitor sensor operation, diagnostic tests and/or remediation processes can be performed on the sensing electrodes of electrochemical gas sensors. Often such processes require the sensor to be out of operation for a length of time due to the time taken to perform the actual process and a subsequent recovery time for the sensing electrode to return to its normal operating state. Such processes include, but are not restricted to, scanning voltammetry to obtain information about sensing electrode activity, or remediation processes such as that described in US201028852A1. It is undesirable for the sensor to be out of operation during such tests.
It is also advantageous to provide means of performing diagnostic tests of various types on electrochemical gas sensors to detect whether the primary gas diffusion access path is operating in the intended mode, or other incorrect/faulty operation modes.
Existing sensor diagnostic tests are often performed by modulating the sensing electrode and monitoring the resulting signal. For example, U.S. Pat. No. 6,251,243 describes a method by which the transient signal resulting from a perturbation to the sensing electrode is used to determine if the sensor is operating correctly. EP 2327981 describes a technique whereby the sensing electrode signal is interrupted to generate a diagnostic. U.S. Pat. No. 5,558,752 and U.S. Pat. No. 6,096,186 describes a means whereby the sensing electrode potential is scanned to measure electrode activity.
Methods such as those described above all potentially suffer from the disadvantage that by perturbing the sensing electrode the sensor may be out of operation for the duration of the test and also may require considerable time (in some cases many hours) to recover back to normal operation following the test. Furthermore, due to the high surface area of typical gas diffusion electrodes any such tests (e.g. scanning voltammetry) need to be performed relatively slowly. This again may result in the sensor being out of use for several hours. As a result such tests can only be performed infrequently, or when the sensor is not in use. For many applications, however, it is desirable to be able to carry out diagnostics more frequently.
Electrochemical gas sensors typically rely on a diffusion limiter such as a membrane or capillary to control access of the target gas to the sensor. There are often also other external restrictions such as protective membranes in the instrument housing. A number of techniques can be used to check the correct internal functioning of such sensors. However they do not test whether the target gas can actually reach the respective sensing electrode and so cannot detect a primary and critical failure mode of electrochemical gas sensors which occurs when such access becomes blocked or restricted. It is therefore desirable to be able to perform a test on an electrochemical gas sensor to ensure that this gas access path is not compromised or blocked, and that the sensing electrode is actually in communication with the ambient air that it is meant to be sampling.
This occurrence may be detected by applying test gas to the sensor, but this is a relatively labour intensive and hence expensive process, particularly for sensors which are located in remote or inaccessible locations. By definition it also requires that the sensor is removed from normal operation, at least for the duration of the test and often for much longer periods to allow for transportation to a test facility.
Many of the existing diagnostic methods such as those described above cannot check the correct operation of the gas access and so offer only partial capture of possible failure modes. There is thus a clear need for improved methods to test whether gas access (capillary, etc.) to toxic sensors (for example) is still functioning, without needing to expose to the target gas.
One solution to this problem uses the fact that, in a CO sensor for example, the platinum sensing electrode can also be used to detect oxygen by running at the appropriate bias potential. This is equivalent to operating the cell as an oxygen pump. Thus, an approach could be to occasionally drive the sensing electrode of a CO sensor to the oxygen reduction potential. The signal generated by oxygen entering through the capillary can be used to check that it is not blocked or restricted, (since oxygen is normally present in the environment). However this is not ideal as the sensor would be out of operation while this was being done and for a significant time afterwards while the electrode recovers back to the operating conditions for CO detection.
Elsewhere in a U.S. Patent Application filed concurrently herewith, and entitled “Auxiliary Micro-electrodes for Diagnostics of Electrochemical Gas Sensors” and assigned U.S. patent application Ser. No. 13/644,485, assigned to the assignee hereof and incorporated herein by reference, we have described methods using a separate, small electrode (microelectrode) or electrodes to perform diagnostics so as to avoid disturbing the operation of the sensing electrode(s) or other electrodes within the sensor. The use of a separate diagnostic electrode avoids interrupting the gas measurement, and the sensor can still operate normally during the diagnostic process.
The above described diagnostic process can be performed quickly and/or continuously by virtue of the fact that the diagnostic electrodes are of a form that allows them to operate without interfering with the other electrodes. However, such approaches cannot meet all the requirements for electrochemical gas sensor diagnostics. For example, there are some measurements of interest in cell diagnostics which cannot adequately be undertaken using a small electrode (microelectrode) due to the low current handling capabilities and consequently low ability to consume target gas.